The correct answer is option 4, Helium. A monatomic element is an element that exists as individual atoms. In other words, it does not form molecules consisting of multiple atoms.

Let`s evaluate the other options to understand why they are not monatomic. Hydrogen (option 1) can exist as H2, which means it is diatomic. It forms a molecule with two hydrogen atoms bonded together. Similarly, sulphur (option 2) can exist as S8, which is an octatomic molecule consisting of eight sulphur atoms. Phosphorus (option 3) can exist as P4, which is a tetraatomic molecule consisting of four phosphorus atoms bonded together.

On the other hand, helium (option 4) does not form molecules. It exists as individual helium atoms. Therefore, helium is considered a monatomic element.

In conclusion, the correct answer is option 4, Helium, because it is the only element listed that exists as individual atoms and does not form molecules.